1. Field of the Invention
The present invention relates to an inverter transformer suitable for an inverter circuit that turns on a light source for lightening a screen of a liquid crystal display.
2. Description of the Related Art
In recent years, a liquid crystal display (hereinafter referred to as xe2x80x9cLCDxe2x80x9d) has been widely employed as a display device for a personal computer or the like. The LCD requires a light source for lightening a screen, which is called xe2x80x9cback lightxe2x80x9d or the like.
Also, in order to brightly illuminate the screen of the LCD of this type four or more cold-cathode fluorescent lamps (hereinafter referred to as xe2x80x9cCFLxe2x80x9d) may be employed as the above light source and made all to discharge and be lightened at the same time.
In general, in order for the CFL of this type to discharge and be lightened, an inverter circuit is employed which receives a d.c. input voltage of about 12 V and generates a high-frequency voltage of about 60 kHz in frequency and about 1600 V in voltage at a secondary side of the Royer oscillating circuit, namely, at a secondary side of an inverter transformer when starting the discharging operation.
The inverter circuit controls the secondary voltage of the inverter transformer and steps it down to about 600 V necessary to keep the CFL discharging after the CFL has started discharging. The voltage control is conducted usually under PWM (pulse width modulation) control.
Up to now, the inverter transformer for use in the inverter circuit has been available in two types, that is, an open magnetic circuit structure using a bar-shaped core as a magnetic core and a closed magnetic circuit structure.
FIG. 9 is a circuit diagram showing an equivalent circuit of an inverter transformer with the open magnetic circuit structure.
In the figure, Ti denotes a step-up ideal transformer of 1:n without any loss, L1 denotes a leakage inductance and Ls denotes an inductance of a secondary winding.
In the inverter transformer with the open magnetic circuit structure shown in the figure, when one CFL is connected to the inverter transformer, the leakage inductances L1, L1 serve as a ballast inductance and the CFL normally discharges with only a very slight voltage drop at an inverter transformer output terminal T.
However, when two CFLs are connected to the inverter transformer, if any one of those CFLs first discharges, the voltage at an output terminal T drops and the other CFL cannot discharge, because the leakage inductances L1, L1 are large.
The inverter transformer with the open magnetic circuit structure using a bar-shaped core as a magnetic core is structured, for example, as shown in FIG. 10, accordingly is simple in structure as compared with the inverter transformer with the closed magnetic circuit structure (not shown) where the magnetic core is formed in a closed shape, for example, a square and a winding must be wound around the magnetic core.
However, with the open magnetic circuit structure, it inevitably happens that the CFL cannot discharge as described above, and therefore one inverter transformer is required for each CFL.
Accordingly, if four or more CFLs are employed for lightening a screen brightly as described above, four or more inverter transformers are necessary. For this reason, there arises such a problem that the overall inverter transformer is large-sized, thereby pushing up the costs.
On the other hand, in the inverter transformer where the magnetic core is formed of the closed magnetic circuit structure, two or more CFLs are connected to one inverter transformer so that all the CFLs can discharge.
However, in this event, if any one of those CFLs discharges and the internal impedance of that CFL is lowered whereby a discharge current flows and a load current increases, then the output terminal voltage of the inverter transformer drops although the closed magnetic circuit structure has a small leakage inductance. This may affect discharging conditions of the other CFLs, resulting in a possible variation in the discharging operation of the respective CFLs.
Also, since the impedance of the CFL has a negative resistant characteristic, if one CFL discharges and is lighted, the impedance of the CFL is rapidly reduced so that a current greatly increases. As a result, the inverter transformer may suffer damages, for example, a broken wire of the winding.
In order to solve the problem that a drop of the output terminal voltage of the inverter transformer with the closed magnetic circuit structure adversely affects the discharging conditions of the other CFLs, ballast capacitors Cb, Cb may be inserted between an output terminal T and the respective CFLs as shown in FIG. 11.
However, the insertion of the ballast capacitors Cb,Cb causes a phase difference in voltage and current resulting in remarkable deterioration in power efficiency and also invites increase in the number of parts and cost.
As described above, the conventional inverter transformer with the open magnetic circuit structure suffers such a problem that the number of inverter transformers increases at a ratio of 1:1 to the number of CFLs whereby the inverter transformers are large-sized as a whole and the costs rise.
Also, the conventional inverter transformer with the closed magnetic circuit structure, although one unit enables a plurality of CFLs to discharge, suffers such a problem that a variation in the discharging operation occurs between the respective CFLs and that the inverter transformer is damaged by excessive-current.
Although the ballast capacitors Cb, Cb may be connected in series with the respective CFLs to prevent the variation in the discharging operation between the respective CFLs, this leads to such a new problem that the power efficiency is deteriorated and that the number of parts and the costs increase.
The present invention has been made in order to provide an inverter transformer with an open magnetic circuit structure, which is free from all of the above problems inherent in a closed magnetic circuit structure and at the same time does not require the number of inverter transformers to increase at a ratio of 1:1 to the number of CFLs thereby downsizing the entire construction as compared with an inverter transformer with a conventional open magnetic circuit structure and preventing an increase in cost.
In order to achieve the above object, according to a first aspect of the present invention, there is provided an inverter transformer, which is provided in an inverter circuit that converts a d.c. voltage into an a.c. voltage and which steps up or down an a.c. voltage inputted to a primary side thereof and outputs a stepped-up or stepped-down voltage to a secondary side thereof, comprising: a plurality of secondary windings; and a primary winding common to the plurality of secondary windings; wherein the respective secondary windings are wound around each of a plurality of bar-shaped magnetic cores, which are formed independently of each other and electromagnetically coupled to the above common primary winding with mutually equivalent characteristics.
According to a second aspect of the present invention, in the first aspect of the present invention, the respective secondary windings are wound around each of the bar-shaped magnetic cores along an axial direction thereof and are divided into a plurality of sections in the axial direction, and an insulating partition plate is disposed respectively between two adjacent sections.
According to a third aspect of the present invention, in the first or second aspect of the present invention, the respective bar-shaped magnetic cores are formed in an L-shape, have the secondary winding wound around one wing of the L-shape, and fixedly positioned relative to the primary winding in such a manner as to be electromagnetically coupled to the primary winding at the entire portion of the other wing of the L-shape.
According to a fourth aspect of the present invention, in any one of the first to third aspects of the present invention, magnetic plates are disposed close to the secondary windings and along the axial direction of the secondary windings in such a manner as to cover both the primary winding and the plurality of secondary windings, or only the plurality of secondary windings.
In the first aspect of the present invention, since the plurality of secondary windings having the mutually equivalent characteristics are provided, and since the primary winding is common to those plural secondary windings, the entire structure is downsized thereby holding down a rise in the costs as compared with the conventional inverter transformer having a plurality (the same number as the secondary) of primary windings as well as secondary windings.
Also, since the open magnetic circuit structure is applied to the present invention, all the problems inherent in the closed magnetic circuit structure can be eliminated such as a variation in the discharging operation between the CFLs connected to the respective secondary windings, a deterioration in the power efficiency caused by adding ballast capacitors, an increase in the number of parts, and the like.
In the second aspect of the present invention, the insulation partition plates disposed respectively between two adjacent sections hold a required creeping distance and prevent creeping discharge.
In the third aspect of the present invention, the bar-shaped magnetic cores with an L-shape, which have the secondary winding wound around one wing of the L-shape, are electromagnetically coupled to the primary winding at the entire portion of the other wings. With this structure, the magnetic flux amount received by the secondary windings from the primary winding is increased as compared with a structure in which the simple bar-shaped magnetic cores are electromagnetically coupled to the primary winding only at the end portions, whereby a higher output can be generated.
In the fourth aspect of the present invention, the value of the leakage inductance can be adjusted in accordance with the arrangement of the magnetic plates which are disposed close to the secondary windings in such a manner as to cover both the primary winding and the plurality of secondary windings, or only the plurality of secondary windings.